Pretreatment process for coating of aluminum materials

ABSTRACT

The invention relates to a method for applying electro-deposited metal coatings ( 3 ) upon aluminium or aluminium alloy components ( 1 ). According to said method, the surface ( 4 ) of the component is cleaned in an appropriate solution, in particular a solution of oils, fats, emulsions, pigments, etc. Said surface ( 4  ) is then etched in an appropriate solution, such that a certain quantity of material or near-surface alloy constituents is dissolved. After cleaning and dissolution, water rinsing is carried out. Immediately after the dissolution of the near-surface regions, the surface ( 4 ) of said component ( 1 ) is activated in a solution, containing iron ions, with a sulphate base by the anodic coupling of said component ( 1 ). The functional layer ( 3 ) is then applied by the cathodic coupling of said component ( 1 ), without intermediate rinsing, in the same electrolyte or in a similar or equivalent electrolyte, said functional layer ( 3 ) being made of iron ( 5 ).

Process for depositing galvanically precipitated metal coatings on components made of aluminum or an aluminum alloy, in which the surface of the component is cleaned in an appropriate solution, particularly of oils, greases, emulsions, pigments, etc., and that the surface is then etched in an appropriate solution, so that a certain amount of the material, or the alloy components near the surface, are dissolved, and that rinsing with water takes place after cleaning and dissolving.

In order to protect components that are subjected to high loads and/or high wear, the components can be subjected various treatments. Treatments to increase the wear resistance include alloying, tempering, and coating. Coating plays a particularly important role for aluminum materials and their alloys, since the positive properties of these materials can be combined with those of the coating.

In engines, and in particular in special tribological systems of piston and cylinder bushings, it has long been standard practice to make these two elements out of aluminum, since the efforts of the engine manufacturers are directed at reducing the weight of the elements. If the piston and cylinder bushings are made of aluminum or aluminum alloys, then the tribological system fails and the contact surfaces show erosion. In order to avoid this erosion and increase the wear resistance of the exposed parts, the state of the technology for many years has been to coat the aluminum pistons.

One problem that occurs in coating aluminum is the highly chemically stable and naturally occurring oxide layer on the surface of the aluminum. In order to improve the adhesion of coatings to the aluminum, or even to make it possible, the oxide layer must be broken down and removed. So that the oxide layer does not regenerate after removal and before any other coating, it is generally typical to apply an intermediate layer to the surface of the aluminum, which then allows precipitation of the so-called functional layers. A functional layer can, for example, consist of iron and serves to improve wear resistance, among other things.

In DE 19 15 762, a process for applying galvanically precipitated metallic coatings to aluminum and aluminum alloys is publicized. The surface of the material is thereby cleaned, then activated and provided with a binding intermediate layer, and finally plated with a coating material. In this case, the coating layer would be the functional layer. The intermediate layers used in this process can be made of zinc, nickel, tin, or copper. The parts to be metallized are placed, after cleaning, in a solution of hydrochloric acid, copper (II) chloride, and a metallic copper powder, until an intermediate layer of pure copper has built up in the bath on the surface of the aluminum.

The disadvantage of this process is the high aggressivity of the chloride electrolyte, so that the use of this process is costly and requires much effort, for example, in the area of operational safety.

The further development of an intermediate layer using zinc is described in the essay: Advances in Zinc Treatment of Aluminum, from the journal JOT, edition 04/2001, by Peter Volk and Dr. Karl Brunn. The essay describes zinc treatment of aluminum as an essential step in the pretreatment for coating aluminum with metals or metal alloys. In the essay, it is noted that processes for direct copper, nickel, or chrome plating have a very narrow process window and cannot be used as stable processes in industrial series production. It is recommended, rather, that pretreatment of the aluminum surface be carried out, by which the surface is activated and the natural oxide layer of the aluminum is removed. Then a thin, conductive intermediate layer is precipitated, which prevents the re-oxidation of the surface during insertion in the coating bath, and enables good adhesion of the coating (functional layer). The further development of the process is directed toward replacing the cyanide-containing zinc agents with cyanide-free substances. This is done by using organic complexes in place of cyanide, and iron in place of nickel and copper. A special complex generation system is developed for cyanide-free zinc agents. The metallic ions are made complex in an exact manner, so that uniform and controlled precipitation, with excellent adhesion, takes place. Simultaneously, the complex generation system allows rapid ion exchange and thereby ensures quick layer buildup. Any indication that an intermediate layer can be avoided completely when precipitating a functional layer, such as the nickel layer described more fully here, is not to be found in the essay. The essay also does not indicate that iron layers can be directly precipitated onto the aluminum surface.

It is therefore the task of this invention to develop a process for coating an aluminum-based material, an aluminum alloy, or a composite material based on aluminum, using a sulfate-based solution, that avoids the use of an intermediate metallic or oxide layer on the surface of the aluminum as a foundation for the precipitation of a functional layer. The coating process is thereby significantly accelerated, and the production costs are simultaneously reduced.

The idea of the invention solves the task at hand in that the component surface is activated, immediately following the dissolution of the areas near the surface in a sulfate solution of iron ions, by anodic switching of the component, and that, in the same or a similar electrolyte, without rinsing in between, the functional layer is deposited by cathodic switching of the component, and that the functional layer consists of iron.

The process in the invention, and the process steps used in it, achieves the goal that the process steps known in the state of the technology, and previously necessary, can be significantly reduced in quantity.

If it was previously generally typical to remove the natural oxide layer from the aluminum, and to precipitate a re-oxidation layer on the aluminum, then this intermediate step and the resulting intermediate layer can be completely avoided.

The components are thereby cleaned of detrimental greases, oils, emulsions, pigments, and similar impurities of the manufacturing process. After cleaning, a thorough rinsing with water takes place. Subsequently, the surface of the component is etched in an appropriate solution; that is, a certain amount of aluminum, or alloy components near the surface, is dissolved. After this, the component is thoroughly rinsed with water. Now, according to the invention, the deposit of an intermediate layer does not take place; rather, the component is immediately placed in a sulfate electrolyte. The typical electrolytes work on the basis of chloride, fluoroborate, or ammonium sulfate. Chloride electrolytes, as indicated above, are very aggressive; the fluoroborates are very aggressive and toxic, and the ammonium sulfate electrolytes are not compatible with sewage systems.

The invention advantageously uses a sulfate electrolyte that is not aggressive, non-toxic, and not damaging to sewage systems. According to the invention, the surface is first activated in the electrolyte by anodic switching with the following process parameters: in a solution with 300 g/l of iron (II) sulfate-heptahydrate (FeSO4*7H2O), at a temperature of 70° C.,

-   -   with a pH value of 2,     -   an activation current density of 2 A/dm², and     -   a treatment duration of 20 seconds.

Subsequently, according to the invention, without rinsing, the functional layer is deposited by cathodic switching of the component, with an iron sulfate electrolyte. This can be done in the same, a duplicate, or a similar electrolyte.

The invention is described below using additional application examples.

In Example 1, hard materials of about 0.5 to 2.0 μm in size are added to the electrolyte. The hard materials could be, for example, aluminum oxide, silicon nitride, chromium nitride, titanium carbide, cubic boron nitride, and diamond particles. The invention refers not only to these listed hard materials, but also includes all solid oxides, oxide ceramics, carbides, and nitrides. These hard materials can, preferably, be used singly, but can also be combined or mixed.

Under the conditions of the invention, an iron layer is deposited that contains about 15% by weight of hard materials. The precipitated functional layer had excellent wear properties and a hardness of approx. 400 HV 0.05.

In a further Example, 2, solid lubricants of about 0.2 to 2.0 μm in size were added to the electrolyte. Solid lubricants, in this case, refers to hexagonal boron nitride, carbon fluoride, graphite, molybdenum sulfide, Teflon, steel particles, or microcapsules filled with oil. These solid lubricants can be used separately or as combinations or mixtures. The experiments showed that the solid lubricants have a most positive influence on the tribological system, and that good values were obtained for friction measurements. A functional layer was deposited in which the solid lubricants were finely dispersed, at a proportion of about 20% by volume.

In a further application of the invention, shown here as Example 3, it is, of course, also possible that both solid lubricants and hard materials are contained in the electrolyte, so that the positive characteristics of these two materials can be combined.

Under these conditions, an iron layer or functional layer is precipitated that contains both materials, finely distributed and dispersed. The hardness of this layer is about 350 HV 0.05.

In the experimental series of Example 4, a component of 5 ml/l of a hypophosphoric acid, such as H3PO2, was added to an electrolyte per the invention, based on sulfates, with 300 g/l iron (II) sulfate heptahydrate. The functional layer created according to the invention process then had a hardness of 700 HV 0.05. With a phosphor component in the electrolyte, it is thereby possible to deliberately influence the hardness of the layer.

In Example 5, a hard material as in Example 1 is added to the phosphor-containing electrolyte from Example 4. The hardness of the layer created was 750 HV 0.05; that is, the hardness could be increased even further. The wear experiments yielded results similar to those in Example 1.

In a further application, Example, 6, of the idea of the invention, a solid lubricant as in Example 2 was added to the phosphor-containing electrolyte of Example 4. Under these conditions, the functional layer was precipitated as an iron layer in which solid lubricants were finely distributed at about 20% by volume.

The hardness of this layer was 650 HV 0.05. The results of the wear experiments were comparable.

In the experiment series of Example 7, mixtures of solid lubricants and hard materials were added to the phosphor-containing electrolyte.

Accordingly, a functional layer was deposited that contained this mixture of materials, finely distributed. The hardness of this layer was 700 HV 0.05. The friction and wear values were comparable to those from Example 4.

The functional layers applied were exceptional in their excellent bond to the base material of the component. A functional layer, applied in accordance with the examples, showed exceptional bonding to the base material in a bond test under extreme conditions, such as thermal shock, glass bead streaming test, and scratch test. The results were comparable to those with a binding intermediate layer on a zinc and copper basis, and were even better in some areas.

The functional layer has an exceptional bond with the base material and is simultaneously the basis for one or more additional layers. The layer materials are named in the sub-claims; however, they are to be interpreted as examples only. Since the functional layer is preferably made of iron, any layer material that has an affinity to iron can be applied on the functional layer. Specifically, these are all metallic materials, but also plastics and ceramics.

The selection of the coating process is also listed in the sub-claims as an example only. In principle, any process that allows iron materials to be coated is suitable. For example, electrochemical, in which tin, copper, etc. can be precipitated; thermal, with which molybdenum, etc.; and reactive processes can be named. Among others worth mentioning are high-velocity flame jet, plasma jet, PVD and CVD processes, as well as screen printing or sprayed-on organic coatings.

Using the process in the invention and the resulting process steps, it is now possible to significantly minimize the number of previously necessary process steps, improve the quality of the products to be coated, reduce production costs, and protect environmental resources.

These advantages allow coating of products with a spectrum of use in market segments that could not previously be served for reasons of cost.

The process of the invention is represented using an application example, and clarified further below. Shown are:

FIG. 1 Cross section through the surface of a component coated as in Example 3 of the description.

In FIG. 1, the cross section through a component coated in accordance with the invention is shown. The figure shows the base material 2 and the functional layer 3 applied to it. In the activation phase the natural oxide layer on the base material 2 was removed, and the components near the surface were dissolved, so that a pure surface 4 was available. On this pure surface 4, using an electrolyte, the functional layer 3 was directly precipitated without an intermediate layer. Per Example 3, the functional layer 3 consists mainly of iron 5; finely distributed hard materials 6 and solid lubricants 7 are dispersed in it. 

1-16. (canceled)
 17. A method of galvanically coating iron on a surface of an aluminum-based substrate, said method comprising: selecting the aluminum component to include an amount of silicon; cleaning the surface of impurities; etching the surface to remove material at the surface; rinsing the surface with water; immersing the surface in an electrolytic iron sulfate solution; and first activating the electrolyte by anodically switching the component and then, without rinsing, depositing a functional layer of the iron by cathodic switching of the component.
 18. The method of claim 17 including selecting FeSO4*7H20 as the electrolytic solution.
 19. The method of claim 18 wherein the electrolytic solution is maintained at a pH value of between 0.5 and 2.5.
 20. The method of claim 19 including selecting the amount of silicon in the aluminum-based component to include between 3 and 22% by weight silicon.
 21. The method of claim 20 wherein the activation of the component in the electrolyte is carried out with an exposure time of between 5 seconds and 5 minutes.
 22. The method of claim 20 wherein the activation of the component and the deposition of the functional layer is carried out with a DC current density of 2 to 20 A/dm².
 23. The method of claim 20 wherein the activation of the component is carried out in the solution maintained at a temperature ranging from 20 to 95° C.
 24. The method of claim 20, including adding to the electrolytic solution particles of at least one hard material having a particle size of about 0.2 to 5 μm and selected from the group consisting essentially of aluminum oxide, silicon nitride, chromium nitride, chromium nitride, titanium carbide, cubic boron nitride, and diamond particles.
 25. The method of claim 20, including adding to the electrolytic solution solid lubricant particles having a particle size of about 0.2 to 5 μm and selected from the group consisting essentially of: hexagonal boron nitride, carbon fluoride, graphite, molybdenum sulfide, Teflon, or microcapsules filled with oil.
 26. The method of claim 20, including adding particles of at least one hard material and at least one solid lubricant to the electrolyte.
 27. The method of claim 20 including adding hypophosphoric acid to the electrolyte.
 28. The method of claim 27 wherein the hypophosphoric acid comprises H3PO2.
 29. The method of claim 28 wherein the H3PO2 is 50% H3PO2 and is added in an amount of 0.25 to 5 ml/l of electrolytic solution.
 30. The method of claim 20 including applying at least one additional layer to the functional layer.
 31. The method of claim 30 wherein the at least one additional layer is selected from the group consisting essentially of: tin, copper, nickel, chromium, of ceramic or metal ceramic materials, and materials and alloys that have an affinity to iron.
 32. The method of claim 31 wherein the at least one additional layer is applied by a process selected from the group consisting essentially of: electrochemical, thermal, and PVD or CVD reactive processes.
 33. The method of claim 20 including selecting a piston as the component.
 34. The method of claim 20 including selecting cylinder bushings as the component. 